The rumblings at Iceland's largest volcano system have raised worries of an eruption that could spell trouble for air travel. In 2010, an ash cloud from the Eyjafjallajokull volcano closed much of Europe's airspace for six days.

"During the night we have had three larger events, two of them in the Bardarbunga caldera. Those were 5.2 and 5.3, and very similar to the events that we have seen there before," said Palmi Erlendsson, a geologist at the Met Office.

He said there had also been a 4.5 magnitude quake at the Askja volcano, 50 kilometers (30 miles)to the north, probably because of magma from Bardarbunga moving in that direction.

The magma still has some 15 kilometres to go before potentially reaching Askja, a scenario which could also cause an ash-producing eruption, like an eruption at Bardarbunga itself, Erlendsson said.

The most likely scenario if there were an eruption now, would be one between the two volcanos, he said.

"That would not be as dangerous ... in terms of ash," he said. "But it's impossible to say what will happen," he added, pointing to the possibility of magma continuing to flow toward Askja, or even of the volcanic activity halting altogether.

The night before saw a magnitude 5.7 quake -- the biggest earthquake yet at Bardarbunga.

On Sunday, Iceland lowered its warning code for possible volcanic disruption to the aviation industry to orange from red, after concluding that seismic activity had not led to a volcanic eruption under the glacier.

A red alert, the highest warning level, indicates an eruption is imminent or underway, with a significant emission of ash likely. (Reporting by Sven Nordenstam; Editing by Crispian Balmer)

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